Infrasound is very low frequency airborne acoustic energy that is inaudible to human beings. Infrasound acoustic waves occupy the frequency band of about 20 Hz to 3.3 mHz. Natural sources of infrasound can include earthquakes, meteors, volcanoes, tsunamis, auroras, and ocean swells. Anthropogenic sources of infrasound can include atmospheric and underground nuclear explosions. Because of its low frequency, infrasound waves experience little attenuation, and can therefore propagate to, and be detectable from, very long distances. Although the signals are inaudible, they may be detected using advanced infrasound sensing technology at ranges of 100s to 1000s of kilometers. The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) operates a worldwide network of about 60 land-based infrasound monitoring stations whose primary purpose is to detect nuclear test explosions. Land-based infrasound sensors can also routinely detect infrasound that is caused by other sources of infrasound at great distances.
From the above, it can be seen that wide global infrasound coverage can be obtained on land using the CTBTO land-based network. However, two thirds of the earth's surface is composed of oceans, and no capability yet exists to monitor infrasound from sensors fielded in the maritime environment (via boat, buoy or Unmanned Surface Vehicle (USV)). The challenges of developing such a capability may be significant; however, overcoming these challenges can provide maritime infrasound coverage where it does not exist, or where it is unreliable due to variable environmental conditions. In addition, event detection redundancy can be achieved by multiple monitoring stations along different propagation paths, which is a desirable capability that could improve event detection confidence, classification information, and localization/tracking performance. Such an expansion of infrasound monitoring capabilities may also provide more complete environmental characterization, which can be important for understanding infrasound performance worldwide. Several technical challenges to operating infrasound sensors in the maritime exist: overcoming heave-induced interference, mitigating noise from wind, forming multi-sensor arrays, and survivability in the harsh, salt water environment. In particular, a solution is needed to overcome to the degrading effects of vertical heave on sensors that are fielded on maritime platforms (boats, buoys, or unmanned surface vehicles, USVs), or that are subject to an undulating ocean surface.
Infrasound monitoring sensors are normally situated on land and consist of micro barometers capable of measuring very small changes in local air pressure. As mentioned above, deployment of an infrasound sensor in the maritime or airborne environment may expose the sensor to motion effects, since the platform is moving with ocean swell and waves in the case of maritime deployment, and with vehicle/platform motion in the case of airborne deployment. The sensor may experience motion along 6 degrees of freedom: surge, sway, yaw, pitch, roll, and heave, However, heave can be the most significant degree of freedom, as even small changes in vertical displacement (heave motion) will induce a change in ambient atmospheric pressure, causing an interference signal against which infrasound signals may be difficult to detect.
In view of the above, it can be an object of the present invention to provide a maritime system which can detect infrasound signals. Another object of the present invention can be to provide a maritime infrasound detection system that can overcome heave-induced interference. Still another object of the present invention can be to provide a maritime infrasound detection system that can account for environmental noise due to rain and wind conditions. Yet another object of the present invention can be to provide a maritime infrasound detection system that can be persistent and autonomous. Another object of the present invention can be to provide a maritime infrasound detection system that can be easily implemented in a cost-effective manner.